A piezo actuated inkjet print head is well known in the art. Such an inkjet print head is commonly provided with a number of ejection units. Each of such ejection units comprises a pressure chamber and a fluidly connected nozzle. The pressure chamber may be filled with a liquid such as an ink and a droplet of the liquid may be expelled through the nozzle by application of a suitable pressure wave in the liquid in the pressure chamber by actuating a piezo actuator that is operatively coupled to the pressure chamber for generating such a pressure wave.
It is also known in the art that the ejection units are sensitive and may become malfunctioning due to a gas bubble, commonly an air bubble, entrapped in the nozzle or pressure chamber. Similarly, dirt or debris may enter the nozzle and cause malfunctioning. Other causes for malfunctioning include liquid residues around the nozzle, electrical failures, drying of the liquid in the nozzle resulting in increased viscosity and deposits of dissolved compounds of the liquid and many more.
When an ejection unit malfunctions, it means that a droplet is not formed correctly. Still after generating a pressure wave, either for droplet ejection or not, a residual pressure wave remains in the liquid and then slowly damps. Characteristic properties of such a residual pressure wave are known to provide detailed information on the cause of the malfunctioning. Therefore and as known, sensing and analyzing such a residual pressure wave may provide detailed information on an operating state of an ejection unit.
In particular, analysis of the residual pressure wave may include comparing the sensed residual pressure wave with a residual pressure wave reference. For example, a residual pressure wave detected from a well functioning ejection unit may be used to determine whether a sensed residual pressure wave corresponds to a well functioning ejection unit. Then, if from an analysis a significant difference between the sensed residual pressure wave and the residual pressure wave reference is derived, it may be concluded that the ejection unit is in a malfunctioning state.
A disadvantage of the above-described known analysis method is that the conditions during which the residual pressure wave is sensed need to be identical to the conditions under which the residual pressure wave reference has been detected. In the prior art, it is therefore known to sense a residual pressure wave when all other ejection units are not actuated, for example, as those other ejection units may cause cross-talk thereby disturbing the sensed residual pressure wave. For example and as a consequence, the detection of an operating state is commonly only performed when the print head is in a non-printing state. However, it is desirable to be able to detect an operating state of an ejection unit also when the print head is in a printing state. More in general, it is desirable to have more flexibility in conditions that are suitable for sensing and analyzing a residual pressure wave.